XMonad.Hooks.ManageDocks: can't toggle dock gap since 0.13

XMonad.Hooks.ManageDocks: can't toggle dock gap since 0.13

Hello,

since version 0.13 of xmonad(-contrib) and upgrading my config to that
version, I find my config not to work as expected. Specifically, my
binding for `sendMessage ToggleStruts` doesn't toggle the dock gap
anymore. Instead, all windows overlap over my dock (dzen2) all the time.

I followed the instructions at the documentation (of
XMonad.Hooks.ManageDocks@xmonad-contrib):